Energy firms have overcharged more than three million customers in the last year, with a total excess bill of £280 million or £78 per household.
The figures, compiled by comparison site uSwitch, indicate that Scottish Power is the worst of the big six energy companies, with more than one in six customers claiming they were sent a wrong bill in the past year.
But uSwitch says bills from E.ON and EDF Energy contain the least number of errors, with less than one in 10 (8%) of their customers stating they have been overcharged in the last 12 months.
Tom Lyon, energy expert at uSwitch.com, says: "It's unacceptable that customers are picking up the cost of suppliers' mistakes. Households are already facing sky-high energy bills and simply can't afford simple blunders."
Almost half (45%) of those incorrectly billed said the problem was a false meter reading, while incorrect Direct Debit amounts accounted for a quarter of people's problems. Over two fifths (43%) of customers had to wait for a fortnight for their billing issue to be resolved - but one in ten were still waiting for a refund at the time uSwitch polled people.
"Correct bills are essential for people to stay in control of their energy and are the least customers should expect," Lyon added. "Recent upgrades by some of the big six to billing systems have resulted in some teething problems, but the industry must do more to tackle these issues and reduce errors.
"We urge customers to always check their bills carefully, and speak immediately to their supplier if they think they have been short-changed. Consumers should also always provide up-to-date meter readings to avoid estimated bills."
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The big six firms and the % of customers who received a higher bill than they should have:
1. ScottishPower - 15.8%
2. SSE - 14.4%
3. npower - 13.9%
4. British Gas - 8.2%
5. EDF Energy - 8.0%
6. E.ON - 7.5%
It has also emerged that more than half the energy customers who switched suppliers in October swapped from the big six firms to smaller suppliers.
The most popular alternative provider was Energy Extra, accounting for more than a third of total switches (36%), according to Comparethemarket.com's energy snapshot.
Its Bright Fixed Price v1 tariff proved a big draw for 12.42% of all switchers, securing them an average annual saving of £306.56 when compared to the average tariff available from the Big Six.
Another two of its deals – Fresh Fixed Price v2 and v3 – were the second and third most popular deals for all energy switchers during the month, accounting for 11.34% and 8.31% of switches respectively.
According to Comparethemarket, Npower saw the biggest proportion of customers leaving for a smaller rival (18.99%), followed by E.ON (18.08%) and Scottish Power (16.91%).
In total, only 3.35% of energy customers switched from a smaller firm to one of the Big Six during October.